The manufacture of iron oxide black pigments on a technical scale is preferably carried out by one of two processes namely the precipitation process or the aniline process (Ullmanns Encyklopadie der technischen Chemie, 4th Edition 1979, page 603, 1.1.3).
Whereas the aniline process starts with metallic iron which is oxidized to iron oxide by nitrobenzene, the starting materials used for the precipitation process are iron-II salts, mainly chlorides and sulphates. The precipitation process may be carried out either as a one stage process or as a two stage process. In the one stage process, air is introduced into the iron salt solution at an elevated temperature and at pH values from slightly acid to slightly alkaline, whereby the divalent iron is directly oxidized to iron oxide black corresponding to the chemical composition of magnetite Fe.sub.3 O.sub.4 and precipitated. The two stage process differs from this one stage process in that oxidation is initially carried out in the acid region until the iron has been oxidized to the stage of goethite, .alpha.-FeOOH, which is then converted into Fe.sub.3 O.sub.4 with freshly precipitated Fe(OH).sub.2 in an aqueous suspension under the conditions of the one stage process.
The iron oxide black suspensions obtained are filtered to isolate the pigment, thoroughly washed, dried and finally ground.
The fineness of grinding varies according to the intended field of application of the pigment, the main criteria being the screen residue and granularity. When iron oxide black pigments are to be used in the building industry, ordinary grinding in conventional roller mills is sufficient. For use in the lacquer and plastics field, finer grinding is frequently desired.
It is known that for grinding inorganic color pigments, greater size reduction can be achieved by using jet mills due to the greater grinding energies produced. Highly superheated steam is frequently used as grinding medium in jet mills. This type of grinding is generally referred to as micronisation (P. Kresse: defazet-aktuell 26, No. 5, 1972, pages 255-259).
Micronisation by steam jet grinding has hitherto not been employed for iron oxide black pigments because of the risk of oxidation by atmospheric oxygen at the mill outlet, which would adversely affect the color quality of the pigments. Experience has shown that iron oxide black pigments cannot be filled into containers in the presence of air without oxidizing to red iron-III oxide unless the temperatures are distinctly below 100.degree. C. Although cooling under an inert gas atmosphere would conceivably be possible, it would be very expensive and difficult.
It has also become known to grind color pigments by micronisation in jet mills using air as grinding medium but jet milling with air has not hitherto been employed industrially for iron oxide black pigments, evidently because it was feared that the high grinding energy would cause the occurrence of active surfaces (R. Schrader: Technik 24, 1969, 2. pp. 88 to 96 or T. G. Burton: Trans. Instn. chem. Engr. 44, 1966, pp. T 37-T 41) which could readily bring about a reaction of the magnetite with the oxygen of the air.
Although it is known from Japan 60-140263 to employ air jet grinding for the micronisation of magnetic iron oxide black which is used, for example, as toner, the iron oxide is in that case protected against oxidation by covering it with waxes and binders which are in any case required for the finished toner.
In the present state of the art, color pigments consisting of iron oxide black and intended for use in lacquers or plastics are obtained by subjecting them to a more intensive grinding in roller mills using a sharper setting for the classifier. The disadvantages of this method lie in the high cost due to the low pigment throughput which is the result of the high residence time required in the mill.
It is an object of the present invention to provide a process for the preparation of improved finely divided iron oxide black pigments which does not have the disadvantages of the processes described.